1. Field of the Invention
The present invention relates to electronic ballasts for powering arc discharge lamps.
2. Background Art
Arc discharge lamps, such as fluorescent tube lamps, are powered by ballasts which limit and otherwise control current to the lamps. Current limiting is necessary because the lamp load appears as a negative impedance to the source causing the lamp to draw an increasing current until either the power supply or the lamp is destroyed. Magnetic ballasts place an inductor in series with the lamp load to limit current. Magnetic ballasts find wide use due to their low cost and reliability. However, magnetic ballasts are bulky, electrically inefficient, prone to emit audible noise, must be selected for a particular lamp load and are not readily dimmable.
Electronic ballasts have been developed to alleviate some of the shortcomings associated with magnetic ballasts. In one type of electronic ballast, AC line voltage is rectified, boosted and commutated to generate a high voltage sinusoidal signal at the same frequency as the line voltage. One difficulty with such designs is that the lamp load voltage must have the same frequency as the line voltage. Another difficulty is that any imperfection in the line voltage waveform is amplified and passed to the lamp load.
Preferably, an electronic ballast should satisfy several, sometimes conflicting, requirements. The electronic ballast should function as a universal ballast. This means that the ballast can drive a wide range of lamp loads. Universal ballasts reduce the need for a multitude of ballast designs, each limited to a specific lamp load.
The electronic ballast should control the AC current supplied to the lamp load. Preferably, the electronic ballast should sense the actual current flowing through the load. This provides increased accuracy in current control, extending lamp life.
In many applications, continuous dimming of the lamp load is desirable. Continuous dimming allows the end user the ability to control the full range of luminous output from the lamp load.
In many electronic ballasts, switching is used in one or more stages. Preferably, this switching will occur at a frequency far enough from the load voltage frequency that effects of switching may be filtered from the lamp power signal. In addition, steps must be taken to ensure that the electronic ballast will not emit electromagnetic interference (EMI) that may affect neighboring devices.
Typically, electronic ballasts generate a balanced AC power signal for the lamp load. Studies have shown, however, that a small amount of DC voltage added to the lamp power signal may prevent flickering during lamp dimming. Therefore, an electronic ballast should have the ability to add a DC voltage to the high voltage AC lamp supply signal.
Electronically ballasted lamps typically appear as an inductive load to the power grid. In installations with many such arc discharge lamps or other inductive loads, it is preferable to correct the lamp power factor so that the lamp load appears more resistive.
Arc discharge lamps are sensitive to spikes in supply voltage. Such spikes or peakedness may shorten the lamp life. The lamp crest factor expresses the ratio of peak voltage to RMS voltage supplied to the lamp. An electronic ballast should maintain the crest value within specified limits. Preferably, lamp crest value control should be separate from power factor control since improvements in one tend to degrade the other.
An electronic ballast should be easily adaptable to a wide range of AC power supplies. Examples of AC supplies include the following: 115 VAC±10%, 400 Hz aircraft line power; 85-265 VAC, 47-66 Hz universal mains; 277 VAC, 47-66 Hz industrial power; 120 VAC, 60 Hz U.S. residential power; 380-800 Hz aircraft wild frequency generator power, and the like.
In addition to these requirements, the electronic ballast should be small, light weight, inexpensive and reliable. The design should pay particular attention to the use of magnetics which can add significant size and weight.